Considerable attention has been given to the use of In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y in long wave emitters such as in DH lasers and detectors. Accordingly, it is well known that devices with improved performance can be fabricated from this material when thin layers or latticed matched layers with a continuously varying composition can be grown.
The methods most frequently used to obtain layers which are thin or of variable composition are molecular beam epitaxy (MBE) or organ metallic vapor phase epitaxy (OMVPE). These two methods are used primarily because low growth rates can be achieved and the nutrient flow can be altered abruptly. Growth rates as low as 02.5 m/h can be readily achieved by MBE simply by lowering the Knudsen cell temperatures while growth can be stopped abruptly with the cell shutter. However, in utilizing traditional MBE techniques, InGaAsP films are difficult to grow because the Arsenine (As)/Phosphorous (P) ratio is hard to control. Moreover, the high vapor pressure of Phosphorous can lead to Phosphorous contamination of the system.
With OMVPE, growth rates as low as 0.5 m/h are easily obtained using dilute gas mixtures while abrupt junctions are achieved by redirecting gas flows to the vent. However, as with MBE, it is also a challenge to grow InGaAsP layers because Phosphorous does not easily incorporate into the lattice and the nutrients can react before they reach the substrate.
Other methods have also been used to grow thin InGaAsP layers, but each has its own set of difficulties. The high growth rates and meltback of liquid phase epitaxy (LPE) make growing thin layers difficult and nonuniformity in the layer growth limits the size of the wafers that can be used. Further, traditional chloride vapor phase epitaxy (VPE) cannot be used because of the long time required for the cation and anion species to reach their steady state concentrations over the source boats. Furthermore, VPE has the additional disadvantage that the cation and anion concentrations cannot be separately controlled.
In contrast to chloride VPE, cation and anion concentrations maybe controlled separately in hydride VPE wherein the anion concentrations can be abruptly and/or continuously changed by varying the AsH.sub.3 and PH.sub.3 flow rates. However, with hydride VPE, control of the cation concentration is still limited by the long time transients between different steady state vapor concentrations.
To remedy some of these deficiencies, variations of the chloride and hydride VPE techniques have been attempted. Generally, these variations have utilized a double barrel reactor in which separate flows are maintained in two different zones and the substrate within the reactor is rapidly rotated between the two flows to grow the thin layers. Using this method, however, results in poor thickness uniformity and the limitation of only two preset compositions.
At present though, there is no method of epitaxially growing thin layers of InGaAsP by hydride VPE which allows the accurate modulation of the composition of InGaAsP alloys. The present invention addresses such a need.